In many industrial processes, control of material thickness is of critical importance, particularly during the manufacturing of layered or coated web material. For example, the manufacturing of photographic film requires the formation of a uniform layer of emulsion on a base support. Non-contact thickness measuring means are preferred so as to not adversely affect the characteristics of the material and to minimize dust, dirt or film residue buildup in a sensor head of a measurement apparatus. Residue buildup on a measurement sensor head would adversely affect the measurement resolution and could result in the loss of data.
Methods for measuring the thickness of a material using interferometers are known in the prior art. For example, U.S. Pat. No. 3,319,515 (Flournoy) relates to the determination of thickness on the basis of interferometric optical phase discrimination. U.S. Pat. No. 5,473,432 (Sorin) and U.S. Pat. No. 5,610,716 (Sorin et al) relate to an apparatus and method for measuring film thickness of a moving film, employing optical reflectrometry. Factors of a moving material, such as vibrations, lateral movements, flutter coupled with angular variation, standing waves, and fluctuations, would render the thickness measurements inaccurate and unintelligible or result in a loss of data. Sorin attempts to account for flutter through the incorporation of an adjustable numerical aperture lens. However, in situations of high flutter, continuous adjustment of the numerical aperture lens is impractical. As the numerical aperture is decreased to tolerate additional flutter, the acceptance angle for reflected light going back into the fiber decreases.
Non-contact thickness measurement methods based on beta, Rutherford backscattering or gamma rays are known to the prior art. These non-contact methods require relatively large beam spot sizes (e.g., one-half inch diameter or larger) to acquire a measurement with sufficient signal-to-noise ratio to provide a useful thickness measurement profile.
Accordingly, a need continues to exist for an apparatus and method for measuring the thickness of a moving material without adversely affecting the material. A need also exists to measure materials with a high degree of lateral resolution. In situations wherein some loss of data may occur, a need further exists to account for the lost data, and to provide a meaningful thickness profile.